Process of making lithopone with high zinc sulphide content



' phide Patented May 3, 1932 UNITED STATES PATENT OFFICE EDWARD A. TAYLOR, OF CLEVELAND, OHIO, ASSIGNOR, BY MESNEASSIGNMENTS, TO

KREBS PIGMENT & COLOR CORPORATION, OF WILMINGTON, DELAWARE, A COR- PORATION OF DELAWARE PROCESS OF MAKING LITHOPON E WITH HIGH ZINC SULPHIDE CONTENT No Drawing.

The term lithopone designates usually the white pigment obtained by calcining a mixture of co-precipitated zinc sulphide and barium sulphate in which the components are present in substantially equimolecular proportions.

It is well known in this art that one essential feature of the process of making this pigment consists in producing the two components simultaneously and from the same reaction medium, and thestandard practice of the precipitation step in making lithopone consists in effecting double decomposition between Water solutions of equimolecular amounts of zinc sulphate and barium sulphide.

While this regular lithopone has for a long time met all the requirements of the trade, it was found that by increasing the zinc sulphide content of such a pigment over that corresponding to the equimolecular proportion improves considerably the covering power of the lithopone. In this case again it is essential that the two components be co-precipitated. These lithopones of high zinc sulphide content are commonly called superlithopones, and for convenience sake this term as used herein is meant to designate lithopones containing more than 1 and not more than 2 molecular equivalents of zinc sulphide to 1 of barium sulphate.

My invention consists now in a process of co-precipitating a mixture of zinc sulphide and barium sulphate in which the molecular ratio of zinosulphide to barium sulphate ranges from 1:1 to 2: 1. This process consists substantially in reacting in aqueous solutions upon zinc sulphate with a barium sulphide compound in which the amount of available S- ions is greater than that of the Ba ions. The barium sulphide compounds corresponding to the above definition are barium hydrosulphide and mixtures of barium sulphide with barium hydrosulphide.

As will'be seen from the formula Ba(SH) barium hydrosulphide has 2 available S" ions against 1 Ba ion, whereas in barium sulphide BaS or if written to correspond to the usual conception of aqueous barium sulsolutions: (Ba(SH) +Ba(OH) Application filed January 23, 1928. Serial No. 248,974.

will otherwise be a waste of material which i cannot react and is lost.

In taking the extreme case of barium hydrosulphide the chemical reactions involved can be represented by the formula:

This reaction equation would appear to be partly reversible as the free sulphuric acid could react with the zinc sulphide formed. I have, however, found that a substantially complete precipitation of this zinc sulphide can be effected if the concentration of the free sulphuric acid in the reaction medium is kept at about or below. It will be necessary when working in higher concentrated solutions to neutralize all of, or at least the sulphuric acid in excess of that corresponding to the above limit concentration. The most convenient neutralizing agents which I can use are the carbonates and bicarbonates of the alkali metals; they do not form insoluble sulphates which would be precipitated together with the zinc sulphide and barium sulphate and contaminate the finished lithopone.

It is evident from the above that by using 1 molecular equivalent of zinc sulphate and 1 molecular equivalent of barium hydrosulphide, a co-precipitated mixture of 1 equivalent of barium sulphate and 2 equivalents of zinc sulphide will be obtained, or, in other words, the finished superlithopone will have substantially the composition 15.5% ZnS plus 54.5% BaSOg this compares with the regular lithopone which has the theoretical composition 29.5% ZnS plus 70.5% BaSO l/V hen a lithopone of a composition intermediate between the above limits is desired, I replace part of the barium hydrosulphide by barium sulphide, and in this manner decrease the amount of S ions available for the precipitation of the zinc sulphide.

The reaction equation can then be expressed by the -following:

w.ZnSO (01-1).Ba(SH) +(2ym).BaS=

The values of no in this formula must, of course, be greater than y and not greater than 2y.

I am aware that ior theoretical reasons it is generally considered that barium sulphide in aqueous solu'tlons exists in aform corresponding to the equilibrium The two components on the right hand side of this equation are, however, not separable and for all practical purposes, particularly the co-precipitation of Zinc sulphide and barium sulphate, the barium sulphide liquor reacts exclusively as if it contained only BaS; as a matter of fact, whatever molecular proportions of barium sulphide and zinc sulphate are allowed to react upon each other, the co-precipitated zinc sulphide and barium sulphate will always appear in equimolecular proportion.

The most convenient procedure for performing my invention is to dissolve the requisite amount of zinc sulphate in water, heat the solution to about 180 F., slowly add thereto the barium hydrosulphide solutions, adding simultaneously the calculated amount of neutralizing agent which, when using soda ash, is added solid and in successive small amounts. It is in this manner very easy to maintain the reaction mixture "faintly alkaline to phenolphthalein or slightly acid, as desired. It is well known that certain properties of the subsequently obtained lithopone can. be controlled by adjusting the acidity or alkalinity of the reaction mixture during the co-precipitation of the Zinc sulphide and barium sulphate.

Similarly, in producing superlithopones of from above 30 to about 45% Zinc sulphide content, a hot aqueoussolution containing the predetermined calculated amounts of barium hydrosulphide and barium sulphide is slowly run into the heated zinc sulphate solution with gradual addition of the neutralizing agent.

The precipitated zinc sulphide and barium sulphate is then filtered off and the green cake dried, calcined and worked up. This part of the lithopone manufacturing process is in no way different when using the superlithopone green cake of my invention, and as it is well known by those skilled in this art and is no part of my invention, need not be described in detail.

The above procedure was substantially followed in the examples given below, which will further illustrate my invention; the parts given therein are by weight:

(1.). 696 parts zinc sulphate solution containing 31.44% zinc sulphate were heated to about 180 F. and a hot mixture of 213 parts barium hydrosulphide solution containing 20% Ba(SH) and 1030 parts barium sulphide solution containing 15.4% BaS slowly introduced, 35 parts soda ash being simultaneously added.

The reagents used are in the proportion 1.44 mols. znSO 0.22 mols. Ba 1 mol. BaS, and035 mols. Na CO The calcined superlithopone had a composition of 30.7 8% ZnS, 0.11% ZnO and 67.75% 132180...

(2). Similarly, the following reagents were used l 1.75 mols. ZnSO 0.38 mols. Ba(SH) 1 mol. BaS, and 0.38 mols. Na 'CO The finished superlithopone contained 32.9% ZnS, 0.08% ZnO, and 64.8% BaSO and had very desirable coloristic as well as physical properties.

(3). 450 parts barium sulphide liquor containing 15.4% BaS, 627 parts barium hydrosulphide liquor containing 20% Ba(sH) 845 parts zinc sulphate liquor containing 31.44% ZnSO and parts soda ash were reacted and the precipitated superlithopone green cake calcined as usual after thorough rking- The finished product had a composition of 38.6% 2 18, 0.09% ZnO, and 60.0% Bas0..

(4) 950 parts barium hydrosulphide liquor containing 20% Ba (SH) 2 were at 150 F. slowly run into 960 parts zinc sulphate liquor containing 31.44% ZnSO and 98.8 parts soda ash gradually added. The green cake was again washed and calcined as usual.

The finished superlithopone contained 42.6% ZnS and 55.75% BaSOa.

It will be apparent from the above that the zinc sulphide in the superlithopone is slightly deficient when compared with the amount expected from the reaction equation. This is partly due to the use of more or less impure, technical products, and also to the well known and diificultly avoidable formation of zinc oxide and other insoluble zinc compounds during the finishing of the lithopone.

I claim:

1. In a process of making a super-lithopone the step comprising co-precipitating zinc sulphide and barium sulphate by reactin upon an aqueous zinc sulphate solution witfi an aqueous solution of a barium sulphide compound in which the amount of available S" ions is substantially greater than the amount of available Ba ions, the amounts of the reacting ingredients being so chosen that the amount of available Zn ions is substantially greater than the said amount of Ba ions.

2. In a process of making a super-lithopone the step comprising co-precipitating zinc sulphide and barium sulphate by reacting upon. an aqueous zincsulphate solution with an aqueous solution of a barium sulphide compound in which the amount of available S" ions is substantially greater than the amount of available Ba ions, the amounts of reacting ingredients being so chosen that the amount of available Zn. ions is substantially equivalent with said amount of S- ions and 

